Fossil Root Casts

The sediments of this terrestrial sandstone were laid down by the swift waters of the ancient Tuolumne River in central California. Sometimes the river laid down thick sandy beds; other times it eroded into earlier deposits. Sometimes the sediment was left alone for a year or more. The dark streaks cutting across the bedding direction are where grasses or other vegetation took root in the river sand. The organic matter in the roots remained behind or attracted iron minerals to leave the dark root casts. The actual soil surfaces above them, however, were eroded away.

The direction of root casts is a strong indicator of up and down in this rock: clearly it was built up in the rightward direction. The amount and distribution of fossil root casts are clues to the ancient riverbed environment. The roots may have formed during a relatively dry period, or perhaps the river channel wandered away for a while in the process called avulsion. Compiling clues like these over a wide region allows a geologist to study paleoenvironments.

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